A solar cell is a device that generates an electromotive force due to minor carriers excited in a positive negative (PN) junction of a semiconductor material by light. Single crystalline silicon, polycrystalline silicon, amorphous silicon or compound semiconductor is used for a solar cell. Although the single crystalline silicon has advantages in energy conversion efficiency, fabrication cost is relatively high. Accordingly, the polycrystalline silicon has been widely used for a solar cell. Recently, a thin film type solar cell including a thin film of amorphous silicon or compound silicon on a substrate of glass or plastic has been suggested.
FIG. 1 is a flow chart showing a method of fabricating a solar cell of crystalline silicon according to the related art, and FIGS. 2A to 2E are cross-sectional views showing a method of fabricating a solar cell of crystalline silicon according to the related art.
At step st11 of FIG. 1 and in FIG. 2A, defects of a positive (P) type substrate 10 generated in a cutting process are removed by a wet etching method using an alkaline solution or an acid solution.
At step st12 of FIG. 1, the substrate 10 is textured for increasing light absorption rate. During the texturing step, minute uneven portions are formed on a surface of the substrate 10. For example, the minute uneven portions may have a pyramid shape. In addition, the texturing step may be performed through a wet etching method using an alkaline solution or an acid solution, or through a dry etching method using a plasma.
At step st13 of FIG. 1 and in FIG. 2B, negative (N) type dopants are diffused in the P type substrate 10 to form a positive negative (PN) junction. For example, a thermal diffusion method may be used for forming the PN unction. A gas including N type dopants such as phosphorus chloride oxide (POCl3) or phosphine (PH3) is supplied to a furnace having the substrate 10 and the N type dopants are diffused into the substrate 10 to form an n+ doped layer 12 on a surface of the substrate 10.
Since the thermal diffusion step is performed at a temperature higher than about 800° C., a by-product such as phosphor silicate glass (PSG) is formed on the surface of the substrate 10 during the thermal diffusion step. Since the PSG blocks a current flow in the solar cell, the PSG is removed at step st14 of FIG. 1 to increase the efficiency of the solar cell. When P type dopants such as boron (B) are used for an N type substrate, a by-product such as boro silicate glass (BSG) is formed on the surface of the substrate. Since the BSG also blocks a current flow in the solar cell, the BSG should be removed.
At step st15 of FIG. 1 and in FIG. 2C, the n+ doped layer 12 in edge portions of the substrate 10 is removed. In the thermal diffusion step, the n+ doped layer 12 is formed in the edge portion of the substrate 10. Since a leakage current between front and rear electrodes is generated through the n+ doped layer 12 in the edge portion, the n+ doped layer 12 in the edge portion of the substrate 10 is removed through an edge isolation step. For example, the n+ doped layer 12 in the edge portion may be cut by a laser. Alternatively, the n+ doped layer 12 in the edge portion may be removed through a wet etching method. In addition, the edge isolation step may be performed before the PSG is removed. Furthermore, the edge isolation may be performed after the solar cell is completed and before the solar cell is tested.
At step st16 of FIG. 1 and in FIG. 2D, an antireflection layer 14 is formed on the n+ doped layer 12. For example, the antireflection layer 14 of silicon nitride (SiN) may be formed through a plasma enhanced chemical vapor deposition (PECVD) method or a sputtering method. The antireflection layer 14 may increase a light absorption rate of the solar cell. In addition, the antireflection layer 14 may protect the surface of the substrate 10 as a surface passivation layer or a hydrogen passivation layer.
At step st17 of FIG. 1 and in FIG. 2E, a conductive paste including aluminum (Al) or silver (Ag) is coated on the front and rear surfaces of the substrate 10 through a screen printing method, and the substrate 10 having the conductive paste is sintered in a furnace of high temperature to form front and rear electrodes 18 and 16 on front and rear surfaces, respectively, of the substrate 10. Specifically while the paste including aluminum (Al) over the rear surface of the P type substrate 10 is sintered aluminum (Al) is diffused into the n+ doped layer 12 on the rear surface of the substrate 10 to form a p+ doped layer 13. The P type substrate 10 and the p+ doped layer 13 form a back surface field in the rear surface of the substrate 10. Due to the back surface field, the electrons excited in the P type substrate 10 by the light are not extinguished at the rear electrode 16. Instead, the electrons move toward the front electrode 18 by the back surface field to contribute to a photoelectric current and improve the light efficiency of the solar cell.
At step st18 of FIG. 1, the solar cell is tested, classified and modularized. The efficiencies of the completed solar cells are tested and the solar cells are classified according to the result of the test. In addition, the solar cells are modularized to form a solar cell module. Before the completed solar cells are tested, the modularization step may be performed. In addition, a laser cutting step for insulation may be performed before the completed solar cells are tested. Further, the edge isolation step of removing the n+ doped layer 12 in the edge portion to prevent a leakage current may be performed before the completed solar cells are tested.
A wet etching method for the edge isolation step has an advantage such that a plurality of substrates are treated at the same time. However, the wet etching method has a disadvantage such that the etching solution causes environmental problems. Recently, a dry etching method using an apparatus for the edge isolation step has been suggested. Since a plurality of substrates are treated one by one in the dry etching method, the dry etching method according to the related art has a disadvantage in productivity. Specifically, since a deposition step of a thin film is performed for a plurality of substrates on a large-sized tray in a fabrication process of the solar cell, the dry etching method treating the plurality of substrates one by one becomes a bottleneck reducing productivity of the fabrication process of the solar cell. Further, since it is required to remove contaminants in edge portions of the substrate is applied to a fabrication process of a semiconductor device or a display device, the edge isolation step has been the subject of recent research and development.